The present invention generally relates to a grating spectrometer, and, in particular, relates to a grating spectrometer including several grating orders.
A conventional grating spectrometer includes a grating arrangement with a diffraction grating, which has a plurality of grating grooves extending closely side by side and onto which an incident light beam enters through an entrance slit, is collimated and is directed by a suitable optical system. The light returned by a diffraction grating is spectrally dispersed. Light of a different wavelengths is returned by angles for which the optical wavelength difference of the light diffracted at the grating grooves amounts to an integral multiple of the wavelength. The light of each wavelength is thus returned at different angles corresponding to optical wavelength differences of, integer, i.e., one, two, three etc., wavelength multiples. An incident white parallel light beam is therefore dispersed into parallel light beams of different colors, i.e., corresponding to different wavelengths, and returned at different angles. Several of such light beams, which correspond to path length differences of the single, double, triple etc. of the wavelength, are thereby associated with each wavelength. These different diffracted light beams of a wavelength are generally called "grating orders". Light at a particular wavelength .lambda. in the first grating order is diffracted by a grating in a particular direction. Light at a wavelength .lambda./2, .lambda./3, .lambda./4 . . . is also diffracted in the same direction as for light at the wavelength .lambda.. These added components are said to be in the second, third, fourth . . . grating order.
The diffracted light beams are focused by an optical system, such that, in the plane of an exit slit, a spectrum is generated which is composed of real images of the entrance slit generated by the light beams of different colors. The exit slit permits passage of light therethrough, which light has been diffracted in a certain direction and which is focused by the optical system at the location of the exit slit. The light emerging through the exit slit is directed to a detector. By rotating the diffraction grating the spectrum is scanned. That is, the light beams diffracted into the different directions can be directed consecutively to the exit slit.
Spectrometers in which the diffracted light beams are focused by the optical system onto a diode array are also known. In this type of spectrometer a plurality of photodiodes are arranged closely side by side. When such a diode array is used the spectrum is not scanned, but the different wavelengths are simultaneously detected and the associated detector signals are output in parallel.
Of course, it is desired to measure only light of one wavelength, i.e., to detect only one grating order. Usually only the first grating order is used. The undesired grating orders thus have to be suppressed. This can be achieved by a cut-off filter or by a prism pre-monochomator. In the UV air is effective as a suitable cut-off filter.
In the first grating order a filter may be used for nearly a complete octave before a higher grating order occurs and the filter has to be changed. A single diffraction grating however may sweep more than two octaves, for example, the spectral range from 190 nm to 900 nm. This large spectral range requires a correspondingly large number of filters, each of which have to be consecutively rotated into the path of rays. Each filter change varies the optical conditions, such that filter steps or filter spikes easily occur in the 100%, or first order-line.
It is known to reduce such filter spikes by stopping the wavelength advance and the pen and by making the filter change during the recording intermission. A considerable expenditure is however required to automatically cause these recording intermissions. Further, the recording time required for scanning a spectrum in the wavelength range is undesirably extended. Finally, the stray light varies stepwise with each filter change. With high sample extinction steps may then easily occur in the recording, even if the 100% or first order-line does not have any steps.
Another disadvantage of a diffraction grating, which is used in the first grating order through a large wavelength range, is the low efficiency of the diffraction grating near the range ends. This reduction of the efficiency often coincides with a decrease of the spectral lamp intensity or of the spectral detector sensitivity.
It is known to use the diffraction grating in a relatively long wave partial range in the first grating order, and in a relatively short wave partial range in a second grating order. This requires, however, a more frequent filter change and the use of band pass filters.
When a diode array is used as a detector, one octave, at best, is detected thereby, and the higher grating orders have to be suppressed by a cut-off filter. As the slopes of the cut-off filter are not infinitely steep, the range, which may be detected by the diode array at once, is even smaller than one octave. In order to measure a larger spectral range with such a detector arrangement it is thus necessary to record the spectrum section by section and to rotate the diffraction grating inbetween.